iii. wet and dry etching
TRANSCRIPT
Applied Physics 298r 1 E. Chen (4-12-2004)
III. Wet and Dry Etching
Vacuum ChamberAtmosphere, BathEnvironment and Equipment
AnisotropicIsotropic
(Except for etching Crystalline Materials)
Directionality
1) High cost, hard to implement
2) low throughput3) Poor selectivity4) Potential radiation damage
1) Inadequate for defining feature size < 1um2) Potential of chemical handling hazards 3) Wafer contamination issues
Disadvantage
1) Capable of defining small feature size (< 100 nm)
1) Low cost, easy to implement2) High etching rate3) Good selectivity for most materials
Advantage
Ion Bombardment or Chemical ReactiveChemical SolutionsMethod
DryWet
Applied Physics 298r 2 E. Chen (4-12-2004)
Pattern Generation (Transfer): Etch vs. Liftoff
Etching Liftoff Mask MaskMask
Etching
StripMask (Resist)
Film
SubstrateLithography
Film
Substrate
Mask
Film
Substrate
Film
LithographySubstrate
Film
Substrate
Mask Mask
Film Film
DepositFilm
Film
SubstrateRemove
Mask (Resist)(Liftoff)
Applied Physics 298r 3 E. Chen (4-12-2004)
Isotropic vs. Anisotropic Etching
Mask
Mask
Mask
RL = 1
0 < RL < 1
RL = 0
Isotropic Etching: Etching rate is the same in both horizontal and vertical direction
Anisotropic Etching: Etching rate is different in horizontal and vertical direction
Lateral Etch Ratio:
( )( )V
HL rRateEtchVertical
rRateEtchHorizontalR =
Isotropic Etching: RL = 1
Anisotropic Etching: 0 < RL < 1
Directional Etching: RL = 0
Bias: the difference in lateral dimensions between the feature on mask and the actually etched pattern
smaller RL results in smaller bias
Applied Physics 298r 4 E. Chen (4-12-2004)
“Under Cut” and “Over Etch”
Bias Bias
Over-Etch results in more vertical profile
but larger bias
Mask
FilmFilm
Mask
Substrate“Under Cut”
Good for Lift-off Substrate
(Rl = 0.5, pattern dimension is better defined)
(Rl = 1, pattern dimension is poorly defined)
Mask
Substrate
Mask
Substrate
Worse in thick filmPoor CD control in
thick film using wet etch
Applied Physics 298r 5 E. Chen (4-12-2004)
Mask Erosion: Film-Mask Etching Selectivity
1) film horizontal etch rate (rfh) < mask horizontal etch rate (rmh):
Film
Mask
Maskθ
Film
( ) ( )mfmf
RSh
W+= θcot2% Substrate
mV
mHmL r
rR = W/2 (Bias)(mask lateral etch ratio)
hf
mV
fVfm r
rS =
(ratio of film and mask vertical etching rate– selectivity)
1 Substrate
Substrate
W/2 (Bias)
Film
Mask
2
2) If film horizontal etch rate (rfh) > mask horizontal etch rate (rmh):
( ) mf
RhW 2% =
Applied Physics 298r 6 E. Chen (4-12-2004)
Film-Mask Selectivity (Sfm) vs. Etching Bias
Selectivity vs Mask Lateral Etch Ratio Rm Selectivity vs Mask Wall angle θ
0
20
40
60
80
100
0 2 4 6 8 10
W/h (%)
Sele
ctiv
ity
θ = 90θ = 60θ = 30
Rm = 100% 0
20
40
60
80
100
0 2 4 6 8 10
W/h (%)
Sele
ctiv
ity
Rm = 100%
Rm = 50%
Rm = 10%
θ = 90º
Most mask material etches isotropically
Selectivity > 20:1
Applied Physics 298r 7 E. Chen (4-12-2004)
Wet Etch Crystalline Materials
• Typically, wet etching is isotropic• However on crystalline materials,
etching rate is typically lower on the more densely packed surface than on that of loosely packed surface
(100)
{110}
{100}
{111}
Si:Diamond Lattice Structure
Surface Atom Density:{111} > {100} > {110}
Etching rate:R(100) ~ 100 x R(111)
(100) Si Wafer
Applied Physics 298r 8 E. Chen (4-12-2004)
Si Wet Etch – (100) Wafer, Mask Aligned in <110> Direction
(100)
{110}
{100}
{111}
(100)
{110}
Mask
(111)54.7º
(100) (100)54.7º
(111)(200-nm size pyramidal pit on (100) Si substrate)
Applied Physics 298r 9 E. Chen (4-12-2004)
Si Wet Etch – (100) Wafer, Mask Aligned in <100> Direction
(100)
{110}
Mask
(100)
{110}
{100}
{111}
<111>
<110>
Etching on (110) Si
<100>
<100>
(Undercut!)
Applied Physics 298r 10 E. Chen (4-12-2004)
GaAs Wet Etch – (100) Wafer
(100)
{110}
{100}
{111}Ga
{111}Ga
{111
}As
{111
}As
<100><100>
(100)
Mask
(100)
Mask
<100>
<100
>
ZincblendeStructure
Etching Rate:
R({111}As)
> R({100})
> R({111}Ga)
Applied Physics 298r 11 E. Chen (4-12-2004)
Typical Wet Etchants
> 50:1Resist~ 500 nm/min1) HCl + H2O2) NaOH
Al
~ 40 nm/min~ 1 um/min
~ 10 um/min
~ 100 nm/min~ 100 nm/min~ 10 nm/min
~ 10 – 1000 nm/min
~ 6 – 600 nm/min (anisotropic)
~ 100 nm/min
Etching Rate
> 50:1Resist1) HCl + HNO3
2) KI + I2 +H2OAu
> 50:1Resist1) H2SO4 + H2O2
+H2O2) Br + CH3OH
GaAs
> 50:1ResistSiO2
1) HF2) BHF3) H3PO4
SI3N4
> 50:1Resist1) HF2) BHF
SiO2
> 50:1Resist1) KOH2) HNO3 + H2O +
HFSi (a-Si)
SelectivityMaskGasMaterial
Applied Physics 298r 12 E. Chen (4-12-2004)
Dry Etching+
Problems with wet etching:Isotropic unable to achieve pattern size smaller than film thickness
Main Purpose of Developing Dry Etching is to Achieve Anisotropic Etching
Type of Dry Etching TechnologyPhysical Sputtering- Physical bombardment
• Ion Mill• Plasma sputtering
Plasma Etching- Plasma-assisted chemical reactionReactive Ion Etching (RIE)- Chemical reaction + ion bombardment
R
+ R
Applied Physics 298r 13 E. Chen (4-12-2004)
Dry Etching Comparison
ChamberPressure
Anisotropy Selectivity
Physical Sputtering(Ion Mill)
• physical bombardment
Reactive Ion Etching• Physical bombardment
+ chemical reaction
Plasma Etching• Plasma assisted chemical reaction
Dry EtchingBeam
Energy
Medium
High
Low
Low10 ~ 100
mtorr
Very Low< 10
mtorr
High> 100Mtorr
Medium
High
Low
Good
Poor
VeryGood
Applied Physics 298r 14 E. Chen (4-12-2004)
Reactive Ion Etching (RIE) • Etching gas is introduced into the chamber
continuously • Plasma is created by RF power• Reactive species (radicals and ions) are
generated in the plasmaradicals: chemical reactionions: bombardment
• Reactive species diffused onto the sample surface
• The species are absorbed by the surface• Chemical reaction occurs, forming volatile
byproduct• Byproduct is desorbed from the surface• Byproduct is exhausted from the chamber
Substrate
e-
Ar
e-
Ar+ t
Shaw Heads
Gases
~ RFGas Selection:1) React with the material to be etched
2) Result in volatile byproduct with low vapor pressure
Applied Physics 298r 15 E. Chen (4-12-2004)
Typical RIE Gases
~ 50:1SI3N4
Metal (Cr, Ni)~ 5001) O2Resist / Polymer
~ 10:1ResistSI3N4
~ 3001) Cl22) BCl3 + Cl2
Al
~ 200
~ 200
~ 100
~ 200
~ 500
Etching Rate (A/min)
~ 40:1SI3N4
Metal (Cr, Ni, Al)1) CH4/H2InP
~ 10:1~ 20:1
SI3N4
Metal (Cr, Ni)1) Cl22) Cl2 + BCl3
GaAs
~ 10:1~ 20:1
ResistMetal (Cr, Ni, Al)
1) CF4 + O2 (H2)2) CHF3
SI3N4
~ 10:1~ 30:1
ResistMetal (Cr, Ni, Al)
1) CHF3 + O22) CF4 + H2
SiO2
~ 20:1~ 40:1
ResistMetal (Cr, Ni, Al)
1) CF42) SF63) BCl2 + Cl2
Si (a-Si)
SelectivityMaskGasMaterial
Applied Physics 298r 16 E. Chen (4-12-2004)
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